1. Field of the Invention
The present invention relates to an exposure apparatus, and an image forming apparatus which uses the exposure apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus generally exposes and scans the surface of an image carrier such as a photosensitive member with a laser beam emitted by a laser light source, thereby forming, on the surface of the image carrier, an electrostatic latent image based on image information. As a known exposure scheme, an image forming apparatus uses, for example, the background exposure (BAE) scheme of exposing based on image information a portion (non-image-forming area) in which no image is to be formed, and not exposing a portion (image-forming area) in which an image is to be formed, on the surface of a charged photosensitive member.
In the BAE image forming apparatus, to maintain a given image quality without generating a density variation in an image obtained by developing an electrostatic latent image on a photosensitive member using a developer (for example, toner), it is necessary to uniform the surface potential (dark and light potentials) of the electrostatic latent image. A technique for uniforming the surface potential of the photosensitive member has been proposed in Japanese Patent Laid-Open No. 2008-275901. In the technique proposed in Japanese Patent Laid-Open No. 2008-275901, a bias current and switching current supplied to a light source are controlled in accordance with a correction value for the sensitivity of the surface of the photosensitive member, thereby controlling the light power of a laser beam emitted by the light source. In light power control described in Japanese Patent Laid-Open No. 2008-275901, a driving current for emitting a laser beam at each of a predetermined target light power and its one fourth is determined by APC (Automatic Power Control) to calculate a light emission start current (threshold current) value based on the determined driving current. Also, the bias current is controlled by adding a current value corresponding to the sensitivity correction value to the calculated light emission start current value.
In the technique described in Japanese Patent Laid-Open No. 2008-275901, laser beam light power control can be performed so as to cancel a potential variation on the surface of the photosensitive member, as long as a laser light emission start current can be calculated precisely. However, if a multibeam laser is used as a light source for exposing and scanning a photosensitive member, it may be impossible to sufficiently reduce a potential variation on the surface of the photosensitive member, as will be described hereinafter.
Note that FIG. 6A is a graph illustrating an example of laser light emitting characteristics representing the relationship between a driving current I of a single laser placed in a laser chip as a light source when the single laser emits a laser beam, and a light power L detected by a photodiode PD placed in the laser chip. The laser slightly emits light without laser oscillation in a region in which the driving current I is zero to a threshold current Ith (exclusive), while it emits and outputs a laser beam with laser oscillation in a region in which the driving current I is equal to or higher than the threshold current Ith, as shown in FIG. 6A. The threshold current Ith can be calculated as the laser light emitting region exhibits linear characteristics, as shown in FIG. 6A, from driving currents IH and IL obtained by APC upon setting a light power Po and its one fourth, respectively, as target light powers.
On the other hand, when APC is performed for a multibeam laser obtained by arranging a plurality of lasers in a laser chip as light sources, it is necessary to perform APC for each of the plurality of lasers. FIG. 6B is a graph illustrating an example (605) of light emitting characteristics obtained by performing APC for one of a plurality of lasers arranged within a laser chip in a multibeam laser. Note that a line 601 shows an example of the actual light emitting characteristics of a laser to undergo APC, and lines 602 to 604 show examples of the light emitting characteristics of three lasers other than the laser to undergo APC when these three lasers emit laser beams without laser oscillation.
When APC can be appropriately performed for the target laser in the multibeam laser, the light emitting characteristics indicated by the line 601 can be obtained. However, it is a common practice in a multibeam laser to, while APC is executed for one of a plurality of lasers, supply bias currents lower than a threshold current to the remaining lasers. The bias currents are supplied to improve the laser light emission response characteristics. In this case, the lasers other than the laser to undergo APC are slightly emitting laser beams (in a bias light emission state) due to the bias currents supplied to them, although they are not in a state of laser oscillation. In such a state, when APC is performed for one target laser upon setting, for example, one fourth of the light power Po as a target light power, the photodiode PD detects the sum of the light power of the target laser and those of the remaining lasers. Hence, a driving current obtained based on the detection result obtained by the photodiode PD may change from the original driving current IL to a driving current IL′, as shown in FIG. 6B. As a result, a threshold current Ith′ (<Ith) lower than the original threshold current Ith by Δ is calculated, that is, an error occurs in the calculated threshold current.
As in this case, if an error occurs in the calculated threshold current, it may be impossible to sufficiently reduce a potential variation on the photosensitive member even when the driving current is corrected using the technique described in Japanese Patent Laid-Open No. 2008-275901 so as to cancel the potential variation. Note that FIG. 6C illustrates an example of how to correct the driving current based on the sensitivity correction value of the photosensitive member. Referring to FIG. 6C, a hatched region 611 shows the amount of driving current to be corrected so as to cancel a potential variation on the photosensitive member. In this case, as shown in FIG. 6C, if a threshold current Ith′ lower than the original value Ith by Δ is calculated, the driving current can be corrected only in an amount indicated by a hatched region 612, so a potential variation corresponding to the amount of driving current, which is indicated by a region 613, remains on the photosensitive member. Therefore, if an error occurs in a threshold current obtained by APC for a multibeam laser, it is difficult to sufficiently reduce a potential variation on a photosensitive member in forming an image by exposure of the photosensitive member.